Preheat your oven to 180°C / 350°F.
Heat the olive oil in a large oven-proof skillet over medium heat.
Add the red onion into the skillet and cook, stirring frequently, until translucent.
Add the corn kernels and give them a good stir to coat with oil, then leave them to cook undisturbed for 2-3 minutes. Stir, and let them cook another 2-3 minutes until lightly browned.
Add the tomatoes to the pan, and stir to combine.
Whisk eggs and milk together, and season with salt and pepper. Add ½ cup of the cheese and the parsley to the egg mixture.
Pour the egg mixture over the corn and tomatoes. Using a heat-proof spatula, gently stir the corn and tomatoes to lift them up.
Sprinkle the top of the frittata with the remaining cheese.
Remove from the stove top and place the frittata into the oven. Bake for 5 minutes, and then switch to the broil setting.
Broil the frittata for about 5 minutes - but watch it carefully, you'll want the cheese to be nicely browned, but look away for too long and it will start to burn.
The frittata is finished when it's puffed and golden, and jiggles only a little bit when you shake the pan.
Remove from oven and let cool for a few minutes before slicing.
Serve with extra parsley and tomatoes.